Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a first image of a display screen or portion thereof with a transitional graphical user interface showing our new design;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof;

FIG. 4 is a front view of a fourth image thereof;

FIG. 5 is a front view of a second embodiment thereof;

FIG. 6 is a front view of a second image thereof;

FIG. 7 is a front view of a third image thereof; and,

FIG. 8 is a front view of a fourth image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-4 and 5-8, respectively. The process or period in which an image transitions to another forms no part of the claimed design.

The broken line showing of a display screen or portion thereof and text and graphical elements forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 